System and method for providing cardioversion therapy and overdrive pacing using an implantable cardiac stimulation device

ABSTRACT

Techniques are provided for coordinating the delivery of cardioversion therapy and overdrive pacing therapy to the heart of a patient, primarily to prevent the re-occurrence of atrial fibrillation (AF) following a cardioversion shock. Included are techniques for modulating the aggressiveness of overdrive pacing by adjusting the magnitude of overdrive pulses or by changing the electrodes with which overdrive pacing pulses are generated. In one example, three phases or “tiers” of AF suppression therapy are provided: cardioversion therapy; far-field dynamic atrial overdrive (DAO) pacing; and near-field DAO pacing. Briefly, a cardioversion shock is delivered to the heart of the patient in response to the detection of AF, then smoothed, far-field overdrive pacing pulses are delivered using widely-spaced electrodes for a period of two minutes while the magnitude of the pulses is gradually reduced. Finally, near-field overdrive pacing pulses are delivered more or les continuously until another episode of AF is detected.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to copending U.S. patent Application Ser.No. 10/374,489, entitled “System and Method for Providing CardioversionTherapy and Overdrive Pacing Using an Implantable Cardiac StimulationDevice,” filed Feb. 25, 2003.

FIELD OF THE INVENTION

The invention generally relates to implantable cardiac stimulationdevices such as implantable cardioverter/defibrillators (ICDs) andpacemakers and, in particular, to techniques for providing cardioversiontherapy and overdrive pacing therapy using implantable cardiacstimulation devices.

BACKGROUND OF THE INVENTION

Atrial fibrillation (AF) is a type of tachycardia wherein the atria ofthe heart beats chaotically, thus interfering with efficient cardiacfunction. Although not fatal, AF can trigger ventricular fibrillation(VF), wherein the ventricles beat chaotically such that there is littleor no net flow of blood from the heart to the brain and other organs.VF, if not terminated, is fatal. Hence, it is highly desirable toprevent or terminate AF and VF.

One technique for preventing AF or VF using a pacemaker is to pacechambers of the heart at a rate somewhat faster than the intrinsic heartrate of the patient using a technique referred to as overdrive pacing.Overdrive pacing may be applied to the atria or the ventricles. Aparticularly effective overdrive pacing technique for the atria,referred to herein as dynamic atrial overdrive (DAO) pacing, isdescribed in U.S. Pat. No. 6,519,493 of Florio et al., entitled “MethodsAnd Apparatus For Overdrive Pacing Heart Tissue Using An ImplantableCardiac Stimulation Device”, issued Feb. 11, 2003. With DAO, theoverdrive pacing rate is controlled to remain generally uniform and, inthe absence of a tachycardia, is adjusted upwardly or downwardly onlyoccasionally. The aggressiveness of overdrive pacing may be modulated byadjusting the overdrive pacing rate and related control parameters. See:U.S. patent application Ser. Nos. 10/093,225 and 10/092,695, both ofFlorio et al., entitled “Method And Apparatus For Using A Rest ModeIndicator To Automatically Adjust Control Parameters Of An ImplantableCardiac Stimulation Device”, and both filed Mar. 6, 2002; U.S. patentapplication Ser. No. 10/043,781, also of Florio et al., entitled “MethodAnd Apparatus For Dynamically Adjusting A Non-Linear Overdrive PacingResponse Function”, filed Jan. 9, 2002; and U.S. patent application Ser.No. 10/043,472, of Falkenberg et al., entitled “Method And Apparatus ForDynamically Adjusting Overdrive Pacing Parameters”, filed Jan. 9, 2002.Capture of overdrive pulses may be verified as set forth in U.S. patentapplication Ser. No. 10/138,438, of Bradley et al., entitled “Method AndApparatus For Providing Atrial AutoCapture In A Dynamic Atrial OverdrivePacing System For Use In An Implantable Cardiac Stimulation Device”,filed May 2, 2002. Each of the aforementioned patent applications isincorporated herein in their entirety.

It is believed that DAO and DVO are effective for at least some patientsfor preventing AF and VF for the following reasons. A normal, healthyheart beats only in response to electrical pulses generated from aportion of the heart referred to as the sinus node. The sinus nodepulses are conducted to the various atria and ventricles of the heartvia certain, normal conduction pathways. In some patients, however,additional portions of the heart also generate electrical pulsesreferred to as “ectopic” pulses. Each pulse, whether a sinus node pulseor an ectopic pulse has a refractory period subsequent thereto duringwhich time the heart tissue is not responsive to any electrical pulses.A combination of sinus pulses and ectopic pulses can result in adispersion of the refractory periods, which, in turn, can trigger atachycardia such as fibrillation. By overdrive pacing the heart at agenerally uniform rate slightly above the intrinsic rate, the likelihoodof the occurrence of ectopic pulses is reduced, the refractory periodswithin the heart tissue are rendered more uniform and periodic and theheart is thereby resynchronized. Hence, the dispersion of refractoryperiods is reduced and the risk of AF or VF is reduced.

Thus, overdrive pacing, particularly DAO and DVO, provides a usefultechnique for helping to prevent the onset of a tachycardia such asfibrillation. As noted, the aggressiveness of overdrive pacing may beadjusted by modulating the overdrive pacing rate and related controlparameters. Circumstances may arise, however, where it is desirable tomodulate the aggressiveness of overdrive pacing by instead adjusting themagnitude of the overdrive pulses or by changing the electrodes withwhich the pacing pulses are delivered to the heart. For example, theremay be a need to ensure that the overdrive pulses are captured in alarger portion of heart tissue so as to achieve enhancedresynchronization. Aspects of the invention are directed to this end. Inparticular, it would be desirable to provide at least two “tiers” ofoverdrive pacing therapy, employing different pulse magnitudes anddifferent electrode combinations, to thereby allow the scope of pulsecapture to be controlled.

Whereas overdrive pacing is primarily directed to preventingtachycardias such as AF from arising, cardioversion is employed toterminate AF once it has occurred. Patients prone to AF may have an ICDimplanted therein that is capable of detecting AF and automaticallyadministering one or more cardioversion shocks in an effort to revertthe atria to a normal sinus rhythm. Typically, the ICD administers abouttwo joules of energy directly to the atria in each cardioversion shock.Cardioversion techniques are described in U.S. Pat. No. 6,445,949 toKroll, entitled “Implantable Cardioversion Device with a Self-AdjustingThreshold for Therapy Selection”, which is incorporated by referenceherein.

Although cardioversion is effective in terminating AF, in many casesfibrillation soon returns, requiring another round of shocks. Repeatedshocks are quite painful and can deplete battery resources of theimplanted device. One reason cardioversion shocks are painful is thatthe patient is typically conscious and alert at the time the shock isadministered. This is in contrast with much stronger defibrillationshocks provided for terminating VF, which are typically not administereduntil the patient has lost consciousness. Because AF is not usuallyimmediately life threatening, painful shocks for its treatment may beperceived by patients as worse than the disease itself and therefore nottolerated. Indeed, anxiety arising in a patient from the fear ofreceiving multiple, painful cardioversion shocks may be sufficient toraise the heart rate sufficiently to trigger such shocks.

As some patients have hundreds of AF episodes annually, it would behighly desirable to provide techniques for preventing the re-occurrenceof AF following a cardioversion shock so as to reduce the need formultiple, repeated cardioversion shocks. It is to this end that otheraspects of the invention are directed. In particular, it would bedesirable to provide techniques for coordinating the delivery ofcardioversion therapy and overdrive pacing therapy so as to prevent there-occurrence of AF following a cardioversion shock.

SUMMARY

In accordance with a first embodiment, a system and method is providedfor delivering both cardioversion therapy and overdrive pacing therapyto the heart of a patient using an implantable cardiac stimulationdevice. Briefly, cardioversion therapy is delivered to the heart using acardioversion shock system, and then overdrive pacing therapy isdelivered, substantially immediately following the cardioversiontherapy, using an overdrive pacing system. By commencing overdrivepacing promptly following cardioversion therapy, the risk of immediatere-occurrence of fibrillation is believed to be substantially reduced.

In accordance with a second embodiment, a system and method is providedfor delivering both far-field and near-field overdrive pacing therapy tothe heart of a patient using an implantable cardiac stimulation device.Far-field therapy (or “global therapy”) is delivered, for example, usingan electrode mounted in the heart in combination with the device body orhousing to ensure that the overdrive pacing pulses are capturedthroughout a large portion of heart tissue to maximize effectiveness.Near-field therapy (or “local therapy”) is delivered, for example, usinga pair of electrodes that are both mounted in the heart, thus providingfor capture within a smaller portion of heart tissue so as to allow forreduced power consumption and reduced patient discomfort. By providingfor both far-field (global) and near-field (local) overdrive pacing, themore aggressive far-field pacing may be performed when needed toresynchronize chambers of the heart, such as immediately followingcardioversion therapy, whereas the less aggressive near-field therapymay be performed at all other times.

In an exemplary embodiment, wherein the implantable device is an ICDconfigured to provide DAO therapy, both aspects of the invention areimplemented, i.e., upon detection of AF, the device providescardioversion therapy, followed by far-field DAO therapy, followed bynear-field DAO therapy. Hence, “triple-tiered” AF therapy is provided.By providing triple-tiered AF therapy, enhanced AF suppression isachieved. Near-field DAO pacing is performed more or less continuouslyto reduce the risk of onset of AF. Should AF nevertheless occur,cardioversion shocks are delivered to terminate AF, then far-field DAOpacing is performed to help resynchronize the atria and prevent animmediate re-occurrence of AF. Far-field DAO is performed for about twominutes immediately following the cardioversion shocks, then near-fieldDAO pacing resumes.

In one particular example, far-field DAO pacing is provided using theright atrial coil electrode in combination with either the devicehousing or ring electrodes mounted in the proximal coronary sinus. Thishelps ensure that the far-field DAO pulses capture throughout most ofthe atria without also pacing the ventricles. Near-field DAO pacing usesotherwise conventional atrial tip-ring electrode combinations. The pulsemagnitudes of the far-field pulses decrease gradually during far-fieldDAO from an initial magnitude such as 20–25 volts down to a lowermagnitude, such as 5–10 volts. Near-field DAO uses only low-magnitudepulses, such as 5 volts. Cardioversion therapy employs one or morecardioversion shocks having smoothed shapes to reduce patientdiscomfort. The far-field DAO pulses also have smoothed shapes to reducepatient discomfort and are each preceded by a pre-pulse inhibition (PPI)pulse provided to further reduce pain. Alternatively, or additionally,transcutaneous electric nerve stimulation (TENS) or neuro-electricacupuncture techniques (NEAP) are employed internally to reduce pain.

Thus various techniques are provided for coordinating the delivery ofcardioversion therapy and overdrive pacing therapy so as to prevent there-occurrence of AF following a cardioversion shock, includingtechniques for modulating the aggressiveness of overdrive pacing byadjusting the magnitude of overdrive pulses and by changing theelectrodes with which the pacing pulses are delivered to the heart.Other features, objects and advantages of the invention are set forthbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention may be more readilyunderstood by reference to the following description taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a simplified diagram illustrating an implantable stimulationdevice in electrical communication with at least three leads implantedinto the heart of a patient for delivering multi-chamber stimulation andshock therapy including cardioversion therapy and overdrive pacingtherapy;

FIG. 2 is a functional block diagram of the implantable cardiacstimulation device of FIG. 1 illustrating basic elements of astimulation device including a tiered AF suppression system forcoordinating the delivery of cardioversion therapy and overdrive pacingtherapy;

FIG. 3 is a functional block diagram of components of the tiered AFsuppression system of FIG. 2;

FIG. 4 is a flow chart providing an overview of the operation of anexemplary embodiment of the invention particularly illustrating themanner by which the tiered AF suppression system of FIG. 3 coordinatesthe delivery of cardioversion therapy and overdrive pacing therapy;

FIG. 5 is a graph illustrating three phases of AF suppression therapyprovided in accordance with the method of FIG. 4 includingcardioversion, far-field DAO and near-field DAO;

FIG. 6 is a flow chart providing individual steps performed by the AFsuppression system of FIG. 3 to implement far-field DAO with PPI pulses;and

FIG. 7 is a graph illustrating far-field DAO pacing with PPI pulsesprovided in accordance with the method of FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description includes the best mode presently contemplatedfor practicing the invention. The description is not to be taken in alimiting sense but is made merely for the purpose of describing thegeneral principles of the invention. The scope of the invention shouldbe ascertained with reference to the issued claims. In the descriptionof the invention that follows, like numerals or reference designatorswill be used to refer to like parts or elements throughout.

Overview of Implantable Device

As shown in FIG. 1, there is a stimulation device 10 in electricalcommunication with the heart 12 of a patient by way of three leads, 20,24 and 30, suitable for delivering multi-chamber stimulation and shocktherapy. To sense atrial cardiac signals and to provide right atrialchamber stimulation therapy, the stimulation device 10 is coupled to animplantable right atrial lead 20 having at least an atrial tip electrode22, which typically is implanted in the right atrial appendage and anatrial ring electrode 23. To sense left atrial and ventricular cardiacsignals and to provide left chamber pacing therapy, the stimulationdevice 10 is coupled to a “coronary sinus” lead 24 designed forplacement in the “coronary sinus region” via the coronary sinus or forpositioning a distal electrode adjacent to the left ventricle and/oradditional electrode(s) adjacent to the left atrium. As used herein, thephrase “coronary sinus region” refers to the vasculature of the leftventricle, including any portion of the coronary sinus, great cardiacvein, left marginal vein, left posterior ventricular vein, middlecardiac vein, and/or small cardiac vein or any other cardiac veinaccessible by the coronary sinus. Accordingly, an exemplary coronarysinus lead 24 is designed to receive atrial and ventricular cardiacsignals and to deliver left ventricular pacing therapy using at least aleft ventricular tip electrode 26, left atrial pacing therapy using atleast a left atrial ring electrode 27, and shocking therapy using atleast a left atrial coil electrode 28.

The stimulation device 10 is also shown in electrical communication withthe heart by way of an implantable right ventricular lead 30 having, inthis embodiment, a right ventricular tip electrode 32, a rightventricular ring electrode 34, a right ventricular (RV) coil electrode36, and an SVC coil electrode 38, also referred to as a right atrialcoil electrode. Typically, the right ventricular lead 30 istransvenously inserted into the heart so as to place the rightventricular tip electrode 32 in the right ventricular apex so that theRV coil electrode is positioned in the right ventricle and the SVC coilelectrode 38 is positioned in the superior vena cava. Accordingly, theright ventricular lead 30 is capable of receiving cardiac signals, anddelivering stimulation in the form of pacing and shock therapy to theright ventricle.

As illustrated in FIG. 2, a simplified block diagram is shown of themulti-chamber implantable stimulation device 10, which is capable oftreating both fast and slow arrhythmias with stimulation therapy,including cardioversion, defibrillation, and pacing stimulation. While aparticular multi-chamber device is shown, this is for illustrationpurposes only, and one of skill in the art could readily duplicate,eliminate or disable the appropriate circuitry in any desiredcombination to provide a device capable of treating the appropriatechamber(s) with cardioversion, defibrillation and pacing stimulation.

The housing 40 for the stimulation device 10, shown schematically inFIG. 2, is often referred to as the “can”, “case” or “case electrode”and may be programmably selected to act as the return electrode for all“unipolar” modes. The housing 40 may further be used as a returnelectrode alone or in combination with one or more of the coilelectrodes, 28, 36 and 38, for shocking purposes. The housing 40 furtherincludes a connector (not shown) having a plurality of terminals, 42,44, 46, 48, 52, 54, 56 and 58 (shown schematically and, for convenience,the names of the electrodes to which they are connected are shown nextto the terminals). As such, to achieve right atrial sensing and pacing,the connector includes at least a right atrial tip terminal (A_(R) TIP)42 adapted for connection to the atrial tip electrode 22. To achieveleft chamber sensing, pacing and shocking, the connector includes atleast a left ventricular tip terminal (V_(L) TIP) 44, a left atrial ringterminal (A_(L) RING) 46, and a left atrial shocking terminal (A_(L)COIL) 48, which are adapted for connection to the left ventricular ringelectrode 26, the left atrial tip electrode 27, and the left atrial coilelectrode 28, respectively. To support right chamber sensing, pacing andshocking, the connector further includes a right ventricular tipterminal (V_(R) TIP) 52, a right ventricular ring terminal (V_(R) RING)54, a right ventricular shocking terminal (R_(V) COIL) 56, and an SVCshocking terminal (SVC COIL) 58, which are adapted for connection to theright ventricular tip electrode 32, right ventricular ring electrode 34,the RV coil electrode 36, and the SVC coil electrode 38, respectively.

At the core of the stimulation device 10 is a programmablemicrocontroller 60, which controls the various modes of stimulationtherapy. As is well known in the art, the microcontroller 60 (alsoreferred to herein as a control unit) typically includes amicroprocessor, or equivalent control circuitry, designed specificallyfor controlling the delivery of stimulation therapy and may furtherinclude RAM or ROM memory, logic and timing circuitry, state machinecircuitry, and I/O circuitry. Typically, the microcontroller 60 includesthe ability to process or monitor input signals (data) as controlled bya program code stored in a designated block of memory. The details ofthe design and operation of the microcontroller 60 are not critical tothe invention. Rather, any suitable microcontroller 6Q may be used thatcarries out the functions described herein. The use ofmicroprocessor-based control circuits for performing timing and dataanalysis functions are well known in the art.

As shown in FIG. 2, an atrial pulse generator 70 and a ventricular pulsegenerator 72 generate pacing stimulation pulses for delivery by theright atrial lead 20, the right ventricular lead 30, and/or the coronarysinus lead 24 via an electrode configuration switch 74. It is understoodthat in order to provide stimulation therapy in each of the fourchambers of the heart, the atrial and ventricular pulse generators, 70and 72, may include dedicated, independent pulse generators, multiplexedpulse generators or shared pulse generators. The pulse generators, 70and 72, are controlled by the microcontroller 60 via appropriate controlsignals, 76 and 78, respectively, to trigger or inhibit the stimulationpulses.

The microcontroller 60 further includes timing control circuitry 79which is used to control the timing of such stimulation pulses (e.g.,pacing rate, atrio-ventricular (AV) delay, atrial interconduction (A—A)delay, or ventricular interconduction (V—V) delay, etc.) as well as tokeep track of the timing of refractory periods, blanking intervals,noise detection windows, evoked response windows, alert intervals,marker channel timing, etc., which is well known in the art. Switch 74includes a plurality of switches for connecting the desired electrodesto the appropriate I/O circuits, thereby providing complete electrodeprogrammability. Accordingly, the switch 74, in response to a controlsignal 80 from the microcontroller 60, determines the polarity of thestimulation pulses (e.g., unipolar, bipolar, combipolar, etc.) byselectively closing the appropriate combination of switches (not shown)as is known in the art. Moreover, as the explained in greater detailbelow, the microcontroller transmits signals to controlling the switchto connect a different set of electrodes during a far-field overdrivepacing than during near-field overdrive pacing.

Atrial sensing circuits 82 and ventricular sensing circuits 84 may alsobe selectively coupled to the right atrial lead 20, coronary sinus lead24, and the right ventricular lead 30, through the switch 74 fordetecting the presence of cardiac activity in each of the four chambersof the heart. Accordingly, the atrial (ATR. SENSE) and ventricular (VTR.SENSE) sensing circuits, 82 and 84, may include dedicated senseamplifiers, multiplexed amplifiers or shared amplifiers. The switch 74determines the “sensing polarity” of the cardiac signal by selectivelyclosing the appropriate switches, as is also known in the art. In thisway, the clinician may program the sensing polarity independent of thestimulation polarity. Each sensing circuit, 82 and 84, preferablyemploys one or more low power, precision amplifiers with programmablegain and/or automatic gain control, bandpass filtering, and a thresholddetection circuit, as known in the art, to selectively sense the cardiacsignal of interest. The automatic gain control enables the device 10 todeal effectively with the difficult problem of sensing the low amplitudesignal characteristics of atrial or ventricular fibrillation. Theoutputs of the atrial and ventricular sensing circuits, 82 and 84, areconnected to the microcontroller 60 which, in turn, are able to triggeror inhibit the atrial and ventricular pulse generators, 70 and 72,respectively, in a demand fashion in response to the absence or presenceof cardiac activity in the appropriate chambers of the heart.

For arrhythmia detection, the device 10 utilizes the atrial andventricular sensing circuits, 82 and 84, to sense cardiac signals todetermine whether a rhythm is physiologic or pathologic. As used herein“sensing” is reserved for the noting of an electrical signal, and“detection” is the processing of these sensed signals and noting thepresence of an arrhythmia. The timing intervals between sensed events(e.g., P-waves, R-waves, and depolarization signals associated withfibrillation which are sometimes referred to as “F-waves” or“Fib-waves”) are then classified by the microcontroller 60 by comparingthem to a predefined rate zone limit (i.e., bradycardia, normal, lowrate VT, high rate VT, and fibrillation rate zones) and various othercharacteristics (e.g., sudden onset, stability, physiologic sensors, andmorphology, etc.) in order to determine the type of remedial therapythat is needed (e.g., bradycardia pacing, antitachycardia pacing,cardioversion shocks or defibrillation shocks).

Cardiac signals are also applied to the inputs of an analog-to-digital(A/D) data acquisition system 90. The data acquisition system 90 isconfigured to acquire intracardiac electrogram signals, convert the rawanalog data into a digital signal, and store the digital signals forlater processing and/or telemetric transmission to an external device102. The data acquisition system 90 is coupled to the right atrial lead20, the coronary sinus lead 24, and the right ventricular lead 30through the switch 74 to sample cardiac signals across any pair ofdesired electrodes.

The microcontroller also includes a tiered AF suppression system 101,which provides for cardioversion therapy, far-field DAO therapy andnear-field DAO therapy. Individual components of AF suppression system101 are shown in FIG. 3 and their operation is described with referenceto FIGS. 4–7. During near-field DAO pacing or whenever DAO pacing is notperformed, the capture of pacing pulses may be automatically verified bymicrocontroller 60. In this regard, any loss of capture of pacing pulsesis detected and backup pulses are delivered. Capture detection ispreferably performed on a beat-by-beat basis. Capture detection istypically not necessary during far-field DAO pacing because thegenerally higher pulse magnitudes used during far-field pacingsubstantially ensure capture. A technique for implementing automaticcapture verification during overdrive pacing is described in theaforementioned patent application of Bradley et al.

The microcontroller 60 is further coupled to a memory 94 by a suitabledata/address bus 96, wherein the programmable operating parameters usedby the microcontroller 60 are stored and modified, as required, in orderto customize the operation of the stimulation device 10 to suit theneeds of a particular patient. Such operating parameters define, forexample, pacing pulse amplitude or magnitude, pulse duration, electrodepolarity, rate, sensitivity, automatic features, arrhythmia detectioncriteria, and the amplitude, waveshape and vector of each shocking pulseto be delivered to the patient's heart 12 within each respective tier oftherapy. Other pacing parameters include base rate, rest rate andcircadian base rate.

Advantageously, the operating parameters of the implantable device 10may be non-invasively programmed into the memory 94 through a telemetrycircuit 100 in telemetric communication with the external device 102,such as a programmer, transtelephonic transceiver or a diagnostic systemanalyzer. The telemetry circuit 100 is activated by the microcontrollerby a control signal 106. The telemetry circuit 100 advantageously allowsintracardiac electrograms and status information relating to theoperation of the device 10 (as contained in the microcontroller 60 ormemory 94) to be sent to the external device 102 through an establishedcommunication link 104. In the preferred embodiment, the stimulationdevice 10 further includes a physiologic sensor 108, commonly referredto as a “rate-responsive” sensor because it is typically used to adjustpacing stimulation rate according to the exercise state of the patient.However, the physiological sensor 108 may further be used to detectchanges in cardiac output, changes in the physiological condition of theheart, or diurnal changes in activity (e.g., detecting sleep and wakestates). Accordingly, the microcontroller 60 responds by adjusting thevarious pacing parameters (such as rate, AV Delay, V—V Delay, etc.) atwhich the atrial and ventricular pulse generators, 70 and 72, generatestimulation pulses. While shown as being included within the stimulationdevice 10, it is to be understood that the physiologic sensor 108 mayalso be external to the stimulation device 10, yet still be implantedwithin or carried by the patient. A common type of rate responsivesensor is an activity sensor, such as an accelerometer or apiezoelectric crystal, which is mounted within the housing 40 of thestimulation device 10. Other types of physiologic sensors are alsoknown, for example, sensors that sense the oxygen content of blood,respiration rate and/or minute ventilation, pH of blood, ventriculargradient, etc. However, any sensor may be used which is capable ofsensing a physiological parameter that corresponds to the exercise stateof the patient.

The stimulation device additionally includes a battery 110, whichprovides operating power to all of the circuits shown in FIG. 2. For thestimulation device 10, which employs shocking therapy, the battery 110must be capable of operating at low current drains for long periods oftime, and then be capable of providing high-current pulses (forcapacitor charging) when the patient requires a shock pulse. The battery110 must also have a predictable discharge characteristic so thatelective replacement time can be detected. Accordingly, the device 10preferably employs lithium/silver vanadium oxide batteries, as is truefor most (if not all) current devices. As further shown in FIG. 2, thedevice 10 is shown as having an impedance measuring circuit 112 which isenabled by the microcontroller 60 via a control signal 114.

In the case where the stimulation device 10 is intended to operate as animplantable cardioverter/defibrillator (ICD) device, it detects theoccurrence of an arrhythmia, and automatically applies an appropriateelectrical shock therapy to the heart aimed at terminating the detectedarrhythmia. To this end, the microcontroller 60 further controls ashocking circuit 116 by way of a control signal 118. The shockingcircuit 116 generates shocking pulses of low (up to 0.5 joules),moderate (0.5–10 joules) or high energy (11 to 40 joules), as controlledby the microcontroller 60. Such shocking pulses are applied to the heartof the patient through at least two shocking electrodes, and as shown inthis embodiment, selected from the left atrial coil electrode 28, the RVcoil electrode 36, and/or the SVC coil electrode 38. As noted above, thehousing 40 may act as an active electrode in combination with the RVelectrode 36, or as part of a split electrical vector using the SVC coilelectrode 38 or the left atrial coil electrode 28 (i.e., using the RVelectrode as a common electrode).

Cardioversion shocks are generated under the control of AF suppressionunit 101. The fibrillation shocks are generated under the control ofother components of microcontroller 60, not separately shown.Cardioversion shocks are generally considered to be of low to moderateenergy level (so as to minimize pain felt by the patient), and/orsynchronized with an R-wave and/or pertaining to the treatment oftachycardia. Defibrillation shocks are generally of moderate to highenergy level (i.e., corresponding to thresholds in the range of 5–40joules), delivered asynchronously (since R-waves may be toodisorganized), and pertaining exclusively to the treatment offibrillation. Accordingly, the microcontroller 60 is capable ofcontrolling the synchronous or asynchronous delivery of the shockingpulses.

As will be explained below, cardioversion shocks and far-field DAOpacing pulses are smoothed to reduce pain. Accordingly, shocking circuit116 includes circuitry for smoothing the cardioversion shocks (as wellas for smoothing defibrillation shocks) and atrial pulse generator 70(as well as ventricular pulse generator 72) likewise includespulse-smoothing circuitry. Shocking circuit 116 and pulse generators 70and 72 additionally include switching circuitry for selectively enablingor disabling pulse smoothing. Techniques for smoothing cardioversionshocks are set forth in U.S. patent application Ser. Nos. 09/967,652 and09/967,647, both of Kroll et al., entitled “System And Method OfGenerating A Low-Pain Multi-Step Defibrillation Waveform For Use In AnImplantable Cardioverter/Defibrillator (ICD)”, and both filed Sep. 28,2001, which are incorporated herein by reference. These and othersmoothing techniques may generally be applied to the smoothing of thefar-field pacing pulses as well.

Referring to the remaining figures, flow charts provide an overview ofthe operation and novel features of stimulation device 10 as configuredin accordance with exemplary embodiments of the invention. In these flowcharts, the various algorithmic steps are summarized in individual“blocks”. Such blocks describe specific actions or decisions made orcarried out as the algorithm proceeds. Where a microcontroller (orequivalent) is employed, the flow charts provide the basis for a“control program” that may be used by such a microcontroller (orequivalent) to effectuate the desired control of the stimulation device.Those skilled in the art may readily write such a control program basedon the flow charts and other descriptions presented herein.

Tiered AF Suppression

FIG. 3 illustrates pertinent components of tiered AF suppression system101 of FIG. 2. The AF suppression system includes an AF detection unit152 for detecting the onset of AF and a cardioversion shock unit 154 fordelivering one or more cardioversion shocks following detection of AF.The suppression system also includes a far-field DAO unit 156 forcontrolling delivery of DAO pacing during a far-field DAO phaseimmediately following the cardioversion shocks and a near-field DAO unitfor controlling delivery of DAO pacing pulses at other times. Thesuppression system also includes a pulse smoothing unit 160 forsmoothing both cardioversion shocks and far-field DAO pacing pulses. ADAO pulse tapering unit 162 provides for a gradual reduction of DAOpacing pulse magnitudes and a PPI unit 164 provides for delivery of PPIpulses prior to other pulses for the purposes of reducing pain. A tieredtherapy controller 166 controls operation of the various components ofthe AF suppression system.

As shown in the FIG. 2, the AF suppression system is preferablyimplemented as a component of microcontroller 60. However, all or aportion of the AF suppression system may be implemented separately fromthe microcontroller. When implemented as a portion of themicrocontroller, the various components of the AF suppression systemoperate to control other components of the implanted device, such asshocking circuit 116 and switch 74, to generate the pacing or shockingpulses, to smooth the pulses, etc. If implemented separately from amicrocontroller, the various components of AF suppression system mayinclude circuitry for actually generating and manipulating the variouspacing and shocking pulses. As can be appreciated, the AF suppressionsystem can be implemented in accordance with a wide range of differentphysical embodiments and no attempt is made herein to illustrate orenumerate all possible embodiments.

Referring to FIGS. 4 and 5, an overview of an exemplary method forachieving AF suppression using the components of FIG. 3 will now bedescribed. Initially, at step 200 of FIG. 4, the AF suppression systemdelivers near-field DAO pacing therapy while monitoring intracardiacelectrogram (IEGM) signals to detect the onset of AF. AF may bedetected, for example, based on the heart rate, i.e., if the atrialheart rate exceeds some threshold value such as 150 beats per minute(bpm), AF is assumed. Other techniques for detecting AF are described inU.S. Pat. No. 6,097,983, to Strandberg, entitled “Cardiac EventDetecting System for a Heart Stimulator”, which is incorporated byreference herein. Near-field DAO is performed in accordance withotherwise standard DAO techniques using relatively low voltage pacingpulses, on the order of 5 volts, delivered between tip and ringelectrodes within the atria, such as right atrial tip electrode 22 andright atrial ring electrode 23. All near-field pacing pulses havesubstantially the same voltage as one another and, because the voltageis relatively low, smoothing is typically not performed.

Briefly, with standard conventional DAO pacing techniques (as described,for example, in U.S. Pat. No. 6,519,493 cited above), the DAO systemdetects the intrinsic atrial rate of the patient then paces the atria ata rate 5 to 10 bpm faster. While overdrive pacing the atria, the systemsearches IEGM signals to detect intrinsic breakthrough beats, i.e.atrial beats that arise despite overdrive pacing. Whenever twoconsecutive atrial breakthrough beats arise, the system increases theoverdrive rate, typically by another 5 bpm. Eventually, if nobreakthrough beats are detected, the overdrive a rate is decremented by,for example, 5 bpm. In this manner, the overdrive rate is maintained, onaverage, slightly above the intrinsic atrial rate of the patient. Asexplained above, this helps prevent the onset of AF and othertachycardias.

If AF nevertheless arises, the AF suppression system delivers one ormore cardioversion shocks, at step 204, and then immediately beginsdelivering far-field DAO pacing therapy, at step 206, for about twominutes before resuming near-field DAO therapy again, at step 200. Thecardioversion shocks delivered at step 204 are preferably smoothed toreduce patient pain. The far-field overdrive pacing pulses are alsopreferably smoothed to reduce patient pain and are gradually reduced inmagnitude from a relatively high voltage such as 25 volts (also referredto herein as the maximum far-field pulse output level) down to a lowervoltage such as 5 volts (also referred to herein as the minimumfar-field pulse output level). Moreover, PPI pain reduction pulses arepreferably delivered prior to each far-field pacing pulse.

Far-field overdrive pacing is delivered using electrodes selected tocapture as much as the atria as possible without unduly pacing theventricles. In one example, far-field pacing is delivered using theright atrial coil electrode with the device housing as the returnelectrode. In another example, a ring electrode 27 or coil electrode 28amounted in the coronary sinus is used as the return electrode incombination with the right atrial coil. By using electrodes that aremore widely spaced than the atrial tip and ring electrodes than used inconnection with near-field pacing and by using greater voltages thanused in connection with near-field pacing, the far-field pacing pulsesthereby capture a larger portion of the atria so as to more effectivelyresynchronize the atria to prevent the reoccurrence of AF during thecrucial period of time immediately following delivery of thecardioversion shock. By smoothing the far-field pulses and by applyingpre-pulse in addition pulses, pain that might otherwise occur duringfar-field pacing is thereby reduced. Far-field overdrive pacing isperformed for between one to three minutes and preferably for about twominutes. Far-field pacing is described in greater detail below withreference to FIGS. 6 and 7.

Hence, three phases or tiers of AF suppression are provided:cardioversion therapy; a far-field DAO pacing therapy; and near-fieldDAO pacing therapy. This sequence is shown in FIG. 5. Briefly, asmoothed cardioversion shock 208 is delivered in response to thedetection of AF (with the AF itself not shown). Then smoothed, far-fieldpacing pulses 210 are delivered while the magnitude or amplitude of thepacing pulses is gradually reduced or tapered. Finally, non-smoothed,non-tapered near-field pacing pulses 212 are delivered during thenear-field phase. Although not shown, during the cardioversion phase twoor more cardioversion shocks may be delivered with potentially differingpulse magnitudes. PPI pulses may be delivered prior to eachcardioversion shock and prior to each far-field pulse. In the unlikelyevent that AF nevertheless reoccurs during far-field DAO pacing,far-field DAO is immediately deactivated and additional cardioversionshocks are delivered.

Within both the far-field phase and the near-field phases shown in FIG.5, pairs of intrinsic breakthrough beats 214 are shown. Prior to thedetection of a pair of breakthrough beats, the overdrive pacing rate isgradually decreased in accordance with the standard DAO pacingtechniques summarized above. Within FIG. 5, the overdrive rate is shownto decrease slightly with each overdrive pacing pulse. In preferredimplementations, however, the overdrive rate only decreases followingsome predetermined number of pacing pulses or following somepredetermined period of time. Finally with regard to FIG. 5, note thatthe shapes of the pulses and shocks shown therein are not intended torepresent the actual shapes of such events but instead provide astylized representation to aid in understanding the invention.

Referring to FIGS. 6 and 7, far-field DAO therapy will now described insomewhat greater detail. Initially, at step 300, the AF suppressionsystem activates or enables the far-field electrode pair for use inperforming far-field DAO pacing, such as the aforementioned combinationof the atrial coil electrode 38 (FIG. 1) and the device housing. Thefar-field electrode pair is enabled by sending control signals toconfiguration switch 74 (FIG. 2) for connecting the selected far-fieldelectrodes with the various pulse generators and sense amplifiers ofFIG. 2. Details regarding techniques for selectively enabling differentelectrode combinations are set forth in U.S. Pat. No. 6,456,876 toKroll, entitled “Dual-Chamber Implantable Cardiac Stimulation System AndDevice With Selectable Arrhythmia Termination Electrode ConfigurationsAnd Method.” Then, at step 302, the system sets the DAO pulse magnitudeto the maximum far-field output level. A far-field DAO timer isactivated, at step 304, which is preset to, for example, two minutes. Atstep 306, the system determines the initial overdrive rate for useduring DAO. The initial DAO rate may be initially determined bydetecting the intrinsic atrial rate of the patient, then setting theoverdrive rate to 5 or 10 bpm higher. Alternatively, since AF has justoccurred, DAO may be programmed to begin at some predetermined highrate, such as 100 bpm.

At step 308, the system delivers smoothed DAO pacing pulses at thecurrent DAO rate with each pulse preceded by a PPI pain reduction pulse.Both the pacing pulses and the PPI pulses are preferably delivered usingthe far-field electrode pairs. Note that the PPI pulses need not bedelivered by the same electrodes as the pacing. However, to beeffective, the PPI should be perceived (but without being painful) andso it is preferably delivered using far-field electrodes. The can is aneffective electrode for the PPI since the chest muscle and skin are verysensitive to electrical stimulation and thus a PPI delivered using thecan as a return electrode is easily perceived. In general, PPI pulsesmay be delivered in accordance with otherwise conventional techniquessuch as those set forth in U.S. Pat. No. 6,438,418 to Swerdlow, et al.entitled “Method and Apparatus for Reduction of Pain from Electric ShockTherapies”. Still other techniques for reducing pain may be employedsuch as those set forth in U.S. Pat. Nos. 5,314,448 and 5,366,485 bothto Kroll et al. Alternatively, or additionally, TENS or NEAP techniquesare employed using the implanted device to reduce pain. The applicationof TENS and NEAP techniques for use within implantable devices isdescribed in U.S. Pat. No. 6,208,902 to Boveja, entitled “Apparatus andMethod for Adjunct (Add-On) Therapy for Pain Syndromes Utilizing anImplantable Lead and an External Stimulator”. Each of the aforementionedpatents is incorporated by reference herein.

At step 310, the system incrementally decreases the pulse magnitude fromthe initial maximum far-field output level down to a minimum far-fieldoutput level, which may be set to, for example, the near-field pulseoutput level. Preferably, the amount of incremental decrease performedat step 310 is selected such that, over a period of two minutes, themagnitude of the far-field DAO pulses decreases graduated from themaximum to the minimum far-field output levels. In any case, assumingthat the far-field timer has not yet expired, processing continues, atstep 306, to determine a new DAO pacing rate and to continue deliveringsmoothed DAO pacing pulses, at step 308. The smoothed far-field pacingpulses 210 are shown in FIG. 7 along with lower-amplitude PPI pulses216. FIG. 7 also shows a pair of intrinsic beats 214, which trigger anautomatic increase in the overdrive rate as explained above. Once thefar-field timer expires, processing returns to FIG. 4. In addition,although not shown in FIG. 6 or 7, should AF nevertheless reoccur duringa far-field overdrive pacing, the overdrive pacing is immediatelysuspended and additional cardioversion shocks are delivered.

Thus, FIGS. 4–7 illustrate exemplary methods for achieving AFsuppression using the components of FIG. 3. Alternative techniques maybe implemented as well. For example, far-field pacing pain reductiontechniques may be selectively activated depending upon the particularelectrode pair that is used for far-field pacing. In this regard, pulsesmoothing and PPI pulses may be activated, for example, only if thedevice housing is used as the return electrode but not otherwise. Inaddition, far-field pain reduction techniques may be deactivated, forexample, once the far-field pulse magnitude has dropped below somepredetermined level, such as 15 volts. Pulse tapering may also beselectively activated, for example, depending upon the particularelectrode pair used for far-field pacing, with tapering only employed ifthe device housing is used as the return electrode. In this regard, ifthe device housing is not used as the return electrode, thensubstantially uniform pulse magnitudes may be employed, for example,throughout far-field pacing with the uniform far-field pulse magnitudesbeing higher than the subsequent near-field levels but lower than theaforementioned maximum far-field pulse magnitude levels. For patientswithout AF, the two levels of DAO pacing may nevertheless be exploited,with the more aggressive far-field DAO pacing employed whenever otheratrial tachycardias have occurred (or there is a risk of suchtachycardias occurring) and with near-field DAO employed otherwise.

As can be appreciated, a wide variety of techniques can be implementedconsistent with the principles the invention and no attempt is madeherein to describe all possible techniques. Moreover, although describedprimarily with reference to atrial overdrive pacing, the techniques ofthe invention may be exploited, modified as needed, for use withventricular overdrive pacing. In addition, although described primarilywith reference to an example wherein the implanted device is adefibrillation/pacer, principles of the invention are applicable toother implantable cardiac stimulation devices as well such as pacemakerswithout defibrillation capability. Moreover, while the invention hasbeen described in the context of dynamic overdrive pacing, it will beapparent to those skilled in the art that the invention may also becarried out using more conventional overdrive pacing techniques. Forexample, the overdrive pacing pulses may be delivered at a presetoverdrive pacing rate, such as 80 ppm or the like.

The various functional components of the exemplary systems may beimplemented using any appropriate technology including, for example,microprocessors running software programs or application specificintegrated circuits (ASICs) executing hard-wired logic operations. Theexemplary embodiments of the invention described herein are merelyillustrative of the invention and should not be construed as limitingthe scope of the invention.

1. In an implantable cardiac stimulation device for implant within apatient, wherein the device has an overdrive pacing system and acardioversion system, wherein the overdrive pacing system includes afar-field overdrive delivery system and a near-field overdrive deliverysystem, a method comprising: delivering cardioversion therapy to theheart of the patient using the cardioversion system; and deliverydynamic overdrive pacing therapy to the heart using the overdrive pacingsystem substantially immediately following the cardioversion therapy,wherein delivering dynamic overdrive pacing therapy comprises:delivering dynamic, far-field overdrive therapy during a first overdrivepacing phase; and delivering dynamic, near-field overdrive therapyduring a second overdrive pacing phase; wherein the first overdrivepacing phase is a predetermined, brief period of time and the secondoverdrive pacing phase is a period of time substantially greater thanthe first overdrive pacing phase.
 2. The method of claim 1 wherein thedevice is connected to a plurality of electrodes and wherein deliveringfar-field overdrive therapy comprises: delivering overdrive pacingpulses using a pair of electrodes selected so that the pacing pulses arecaptured throughout substantially all of the atria.
 3. The method ofclaim 2 wherein the device includes a housing and wherein the pluralityof electrodes include a right atrial coil electrode and whereindelivering far-field overdrive therapy comprises: delivering overdrivepacing pulses between the right atrial coil electrode and the devicehousing.
 4. The method of claim 2 wherein the plurality of electrodesinclude a right atrial coil electrode and a ring electrode mounted nearthe proximal coronary sinus and wherein delivering far-field overdrivetherapy comprises: delivering overdrive pacing pulses between the rightatrial coil electrode and the ring electrode mounted near the proximalcoronary sinus.
 5. The method of claim 1 wherein delivering far-fieldoverdrive therapy comprises: delivering overdrive pacing pulses whilegradually reducing a magnitude of the pacing pulses.
 6. The method ofclaim 1 wherein delivering far-field overdrive therapy comprises:delivering overdrive pacing pulses having a pulse shape selected toreduce patient pain.
 7. The method of claim 6 wherein deliveringoverdrive pacing pulses having a pulse shape selected to reduce patientpain comprises: delivering smoothed overdrive pacing pulses.
 8. Themethod of claim 1 wherein delivering far-field overdrive therapycomprises: delivering a series of pairs of pulses each including apre-pulse inhibition (PPI) pulse and a subsequent overdrive pacingpulse.
 9. The method of claim 1 wherein delivering far-field overdrivetherapy during a first overdrive phase comprises: delivering far-fieldoverdrive therapy for between one and three minutes.
 10. The method ofclaim 1 wherein delivering dynamic overdrive pacing therapy comprises:delivering dynamic atrial overdrive (DAO) therapy.
 11. An implantablecardiac stimulation device for implant within a patient, a systemcomprising: a cardioversion shock delivery system; a dynamic overdrivepacing system including a far-field overdrive pacing system and anear-field overdrive pacing system; and a control system operative tocontrol the cardioversion system to generate a cardioversion shock fordelivery to the heart of the patient and to control the dynamicoverdrive pacing system to generate dynamic overdrive pulses fordelivery to the heart of the patient substantially immediately followingthe cardioversion shock and to control the far-field overdrive pacingsystem to deliver far-field overdrive pacing therapy for apredetermined, brief period of time and to control the near-fieldoverdrive pacing system to deliver near-field overdrive pacing therapyfor a period of time substantially greater than the predetermined, briefperiod of time.
 12. In an implantable cardiac stimulation device forimplant within a patient, wherein the device has a far-field overdrivepacing system, a near-field overdrive pacing system and a cardioversionsystem, a method comprising: delivering cardioversion therapy to theheart of the patient using the cardioversion system; deliveringfar-field overdrive pacing therapy to the heart using the far-fieldoverdrive pacing system following the cardioversion therapy; anddelivering near-field overdrive pacing therapy to the heart using thenear-field overdrive pacing system following the far-field overdrivepacing therapy; wherein the far-field overdrive pacing therapy isdelivered for a predetermined, brief period of time and the near-fieldoverdrive pacing therapy is delivered for a period of time substantiallygreater than the predetermined, brief period of time.
 13. The method ofclaim 12 wherein the device is connected to a plurality of electrodesand wherein delivering far-field overdrive therapy comprises: deliveringoverdrive pacing pulses using a pair of electrodes selected so that thepacing pulses are captured throughout substantially all of the atria.14. The method of claim 13 wherein the device includes a housing andwherein the plurality of electrodes include a right atrial coilelectrode and wherein delivering far-field overdrive therapy comprises:delivering overdrive pacing pulses between the right atrial coilelectrode and the device housing.
 15. The method of claim 13 wherein theplurality of electrodes include a right atrial coil electrode and a ringelectrode mounted near the proximal coronary sinus and whereindelivering far-field overdrive therapy comprise: delivering overdrivepacing pulses between the right atrial coil electrode and the ringelectrode mounted near the proximal coronary sinus.
 16. An implantablecardiac stimulation device for implant within a patient, a systemcomprising: a lead system configured for implant within the patient; acardioversion shock delivery system operative to deliver a cardioversionshock via the lead system; a far-field overdrive pacing system operativeto generate overdrive pacing pulses for far-field delivery to thepatient via the lead system; a near-field overdrive pacing systemoperative to generate overdrive pacing pulses for near-field delivery tothe patient via the lead system; and a control system operative tocontrol the cardioversion system to generate a cardioversion shock fordelivery to the heart of the patient and to control the far-fieldoverdrive pacing system to deliver far-field overdrive pacing therapyfor a predetermined, brief period of time to the heart of the patientfollowing the cardioversion shock and to control the near-fieldoverdrive pacing system to deliver near-field overdrive pacing therapyfor a period of time substantially greater than the predetermined, briefperiod of time to the heart of the patient following the far-fieldtherapy.
 17. The device of claim 16, wherein: the far-field overdrivepacing system is operative to generate dynamic overdrive pacing pulses.